Downloads mail from a server to a single client, traditionally deleting the server copy. Largely superseded by IMAP, which suits multiple devices far better.
Plain text unless upgraded with STARTTLS. Prefer POP3S on 995, or better, IMAP.
Open 110 usually means an old mail configuration nobody has revisited. Move clients to 995 or to IMAP and close it, since plain POP3 exposes passwords.
The port scanner will tell you whether 110 is reachable on a host you control. Checking from outside your own network matters: a port can be open on the machine and still be blocked at the firewall, and it is the view from the internet that decides whether anyone else can reach it.
Only scan hosts you own or have permission to test.